Enterotoxigenic Escherichia coli (ETEC) is a Gram-negative bacterium that is responsible for between 280 million and 400 million episodes of diarrhoea, and approximately 380,000 deaths per year (Taxt et al. Infect Immun. 2010 May; 78(5):1824-31. Epub 2010 Mar. 15). Most of the victims are children of less than 5 years of age living in developing countries. In addition, ETEC is considered the most common cause of traveller's diarrhoea.
ETEC colonises the small intestine and is transmitted by the faecal-oral route. The bacteria adhere to the intestinal epithelium via characteristic colonisation factors (CFs). CFs are fimbriae or fibrillae-filamentous proteins that are presented on the bacterial surface. To date, 25 distinct ETEC CFs have been identified (Rivera et al., J Clin Microbiol. 2010 September; 48(9): 3198-3203).
Numerous attempts have been made at developing an ETEC vaccine but no broadly effective vaccines are available to protect humans against ETEC diarrhea. A killed whole-cell vaccine represents the most promising candidate to date (Jertborn et al. Vaccine 1998; 16:255-60, Sack et al. Vaccine, 2007 May 30; 25(22):4392-400. Epub 2007 Apr. 4). This comprises five ETEC strains that are co-administered with recombinant cholera toxin B subunit (CT-B). The five ETEC strains were selected to provide the most common CFs.
Enterotoxins known as Heat Stable (ST) and heat-labile (LT) toxins, in addition to CFs are virulence determinants in ETEC diarrhoea. Both toxins act by stimulating net secretion of ions and water by intestinal epithelial cells. This causes watery diarrhoea, which can lead to a cholera-like condition in the most extreme cases.
ST was identified as target for an ETEC vaccine in the early 1980s but, despite many attempts, no successful ST-based vaccine has been developed.
Three main problems underlie the difficulty in utilising ST as a vaccine component. First, the ST polypeptide is inherently toxic. Second, in its natural form, the ST polypeptide is non-immunogenic. Finally, ST closely resembles the endogenous polypeptides guanylin and uroguanylin, which raises the possibility that anti-heat-stable toxin antibodies may cross-react, and cause autoimmune disorders in vaccinees.
Several ST mutants have been reported to reduce toxicity (Taxt, A. et al. Infect. Immun. 78, 1824-1831 (2010) and Liu, M. et al. Toxins 3, 1146-1162 (2011). However, as ST is a small polypeptide, most mutants with reduced toxicity also have reduced immunogenicity.
The problem of providing a heat-stable toxin mutant that exhibits both reduced toxicity and specific immunogenicity remains a significant challenge. Despite numerous attempts, no successful heat-stable toxin vaccine component has been developed that elicits a neutralising and protective immune response.